1. Field of the Invention
The invention relates to a signal transmission system comprising a receiving device which includes a carrier synchronizer means for estimating and compensating for synchronization errors between a local carrier and a modulated input signal, while the device further includes an input signal demodulator which produces a demodulated signal, and a processor for processing the demodulated signal, the synchronizer comprising a first correction loop which includes, on the one hand, a phase/frequency detection detector for producing a first error signal, a first low-pass filter which has a first passband for filtering the first error signal, which filtered first error signal corrects the synchronization errors in the unlocked mode, and, on the other hand, a generator for generating the local carrier, and a second correction loop which includes a phase detector which produces a second error signal which corrects the synchronization errors in the locked mode, and a controller for controlling the loops and for putting the synchronizer either in the unlocked mode or in the locked mode.
2. Description of the Related Art
The invention also relates to a receiving device used especially in systems which utilize coded modulations via phase jumps (PSK) or via amplitude jumps and via phase jumps (QAM). Such systems may be used, for example, in digital television or telephony.
Carrier recovery circuits are known which utilize a carrier recovery loop which includes an oscillator whose voltage is controlled by an error signal that comes from the phase/frequency difference which indicates the out-of-sync between the local carrier produced by the oscillator and the transmit carrier.
A circuit of this type, but more sophisticated, is described, for example, in the document U.S. Pat. No. 4,794,623 which relates to a recovery of a carrier signal. For rapidly obtaining the frequency and the phase of the transmit carrier signal, this circuit utilizes two recovery channels which adjust the frequency and phase of a local oscillator to the transmit carrier signal. In a first period of time a phase/frequency detector appears for locking on to the local oscillator after which, in a second period of time, a phase detector effects a finer phase synchronization.
In addition, it is known that in a receiving device there is a drawback in that there is a delay of reaction in the carrier recovery loop, which delay elapses between the instant at which the input signal appears on the input of the device and the instant at which the processed input signals arrive which are necessary for the operation of the phase/frequency detector. This delay, which may be very long, is due to the processing of the input signal, such as in the demodulation, filtering, equalization, decision steps. The existence of such a delay leads to selecting the parameters of a carrier recovery loop by making compromises between various constraints of the system. Such a compromise is made, for example, between the maximum permissible value of the phase jitter and the minimum value of the natural frequency of the loop, which frequency occurs in the range of the acquisition domain and with respect to the robustness of the loop in respect of phase noise which is caused, for example, by microphonic effects.